An electroluminescent device is generally prepared by forming a transparent electrode, a lower insulating layer, a luminescent layer, an upper insulating layer and a back electrode on a transparent substrate, made of glass and the like, in this order. In this process, the transparent electrode, the lower and upper insulating layers are formed by sputtering, and the luminescent layer and the back electrode are formed by electron beam vapor deposition.
As mentioned above, since two processes are employed for preparing one device, the change of apparatus and other treatments therewith are required, thus increasing the production cost of the device. In addition, when the luminescent layer is formed by an electron beam vapor deposition method, a layer having very poor crystallinity, a so called "dead layer", is initially formed. Accordingly, in order to obtain high luminance, the luminescent layer must be increased in thickness so as to increase a driving voltage.